An interferometer observing the sky measures voltage fluctuations of the incoming electromagnetic wave. The Van Cittert-Zernike theorem expresses the relationship between the incoming radiation and the actual measured quantity as [1]}:
\(V ({\mathbf {U}},\nu ) = \int \int A(\hat{\mathbf {s}},\nu ) B(\nu ) I(\hat{\mathbf {s}},\nu ) e^{-i2\pi \nu \mathbf {U}.\hat{\mathbf {s}}} d\Omega ,\) 
